This invention relates to conductivity meters and, in particular, to a meter for accurately and rapidly measuring the conductivity of dialysate.
In the human body, the products of biochemical reactions, known as metabolites, are removed from the blood by diffusion from the blood into the kidneys. If the kidneys fail to function properly, the concentration of metabolites in the blood increases, eventually reaching toxic levels. Metabolites can be removed from the blood by apparatus, known as a hemodialyzer, in which diffusion of metabolites takes place through a semi-permeable membrane separating the blood from an aqueous salt solution known as dialysate.
In general, diffusion depends upon concentration gradient, the difference in concentration between one region and another. Molecules diffuse from a region of higher concentration to a region of lower concentration. A semi-permeable membrane permits only molecules smaller than a predetermined size to pass through the membrane. In a hemodialyzer, the blood is on one side of a membrane and the dialysate is on the other side of the membrane. The dialysate is pumped past the membrane much more quickly than the blood to assure a continuous, fresh supply of dialysate and a high concentration gradient across the membrane, i.e. a very low concentration of metabolites in the dialysate.
A semi-permeable membrane is a two-way street for those molecules which can pass through it. Therefore, the concentration of salts in the dialysate must be carefully monitored to assure that it matches the concentration of salts in the blood, otherwise salts may be added to or removed from the blood unintentionally.
A hemodialysis system produces dialysate by diluting a concentrated salt solution with pure water. The dialysate approximates the salts in human blood, with some salts in higher or lower concentration as determined by a patient's physician. That is, hemodialysis may include adding certain salts, and/or glucose, to the patient's blood during treatment, in addition to removing metabolites from the patient's blood.
Aqueous solutions, particularly aqueous salt solutions, conduct electricity in varying degrees, depending upon the particular salts and the concentration of salt dissolved in the solution. The concentration of salts in dialysate is therefore usually monitored by measuring the conductivity of the dialysate. The pH of the dialysate may be monitored as well. Despite the control systems built into hemodialysis systems, manufacturers typically recommend that the concentration of salt in the dialysate be checked just prior to each patient's hemodialysis.
As known in the art, the conductivity of aqueous solutions is strongly temperature dependent, varying as much as four percent per degree centigrade. For dialysate, the variation is approximately two percent per degree centigrade. Thus, measuring conductivity is not sufficient: one must also measure the temperature of the solution and correct the measurement of conductivity for temperature. Circuitry for measuring conductivity and correcting for temperature is known in the art, e.g. analogue multipliers, analogue to digital converters having a reference voltage which varies with temperature, or a look-up table stored in a memory accessed by a microprocessor. Conductivity is usually expressed as Siemens per square centimeter at 25.degree. C.
For the specific case of dialysate, it is desirable to know the conductivity very accurately. The problem is that making the measurement affects the measurement; i.e. the temperature of the meter can affect the measurement if the temperature of the meter is not the same as the temperature of the dialysate.
U.S. Pat. No. 4,553,552 to Valdespino et al. is premised on an "instantaneous" reading obviating the need to measure temperature, although no experimental data is disclosed substantiating the premise. In the Valdespino et al. patent, the conductivity of dialysate is measured in a syringe filled through a needle inserted into the hemodialysis bath. Because of the large variation in conductivity with temperature, the problem remains of accurately and rapidly measuring the conductivity of dialysate.
In view of the foregoing, it is therefore an object of the invention to provide an improved meter for measuring the conductivity of dialysate.
Another object of the invention is to provide a meter for accurately and rapidly measuring conductivity, thereby minimizing the effect of temperature on the measurement.
A further object of the invention is to provide a flow-through conductivity meter.
Another object of the invention is to provide a conductivity meter which can be left on-line or used separately as a hand-held instrument.
A further object of the invention is to provide an improved cell for measuring the conductivity of a liquid.